Mirroring may be performed in a storage system for which high reliability is needed. A mirroring technique is used so that data duplication will be performed between two hard disk drives. Even if a failure occurs in one hard disk drive, data can be read out from the other hard disk drive by performing mirroring. This prevents the data from being lost (from not being restored).
However, even if data duplication is performed, the following problem arises. If failures occur simultaneously in the two hard disk drives between which data duplication is performed, then the data is lost. Failures rarely occur simultaneously in the two hard disk drives from different causes. It is assumed that failures occur simultaneously in the two hard disk drives. In many cases, the failures are caused by a problem which arises at the time of manufacturing the two hard disk drives.
Accordingly, the following method is proposed. In order to realize data redundancy, a magnetic storage device and a semiconductor memory device are used for forming a mirroring pair. By realizing data redundancy by the use of storage apparatus of different types, simultaneous failures from the same cause can be prevented (see, for example, Japanese Laid-open Patent Publication No. 2006-260008).
By the way, if mirroring is performed between storage apparatus of different types, data access speed depends on a lower-performance storage apparatus. As a result, efficiency in data access falls. For example, it is assumed that mirroring is performed between a magnetic hard disk drive and a semiconductor memory device. Data stored on the semiconductor memory device can be accessed at a high speed. However, data access cannot be completed until access to data stored on the magnetic hard disk drive is completed. That is to say, the data access performance of the semiconductor memory device is not demonstrated and its capability is not utilized effectively.
To form a mirroring pair by the use of two storage apparatus which are equal in performance is fundamental in performing mirroring while utilizing the capability of each storage apparatus effectively. Therefore, each ordinary redundant array of independent disks (RAID) system having a mirroring function includes the same model of a plurality of hard disk drives manufactured by the same enterprise. The same model of hard disk drives are equal in data access performance and their performance can be demonstrated sufficiently.
However, the same failures may occur in industrial products, such as hard disk drives, manufactured as a specific lot at a factory. Usually such failures are referred to as lot failures. Such lot failures occur not only in hard disk drives but also in all the other storage apparatus including semiconductor memory devices. If lot failures occur in storage apparatus manufactured as a specific lot at a factory and mirroring is performed between two storage apparatus included in the lot, then there is a very high probability that the two storage apparatus will break down simultaneously.